1. Field of the Invention
The present invention relates to a water-insoluble hydrophilic polymer composition and, more specifically, to a water-insoluble hydrophilic polymer composition of a polymer having tertiary amino groups and carboxyl groups and an epoxy compound, and which is capable of crosslinking at ambient temperature to provide a polymer having water absorption rate adjustable over a wide range and which is highly water-proof to provide an excellent coating strength in water.
2. Description of the Prior Art
Since water-insoluble hydrophilic resin compositions are both water-absorptive and water-proof, a wide variety of applications have been developed for them such as anti-fogging coating compositions and materials for various medical appliances, a noticeable application among which is their use for coating underwater structures. It is disclosed, for example, in U.S. Pat. No. 3,896,753 and U.S. Pat. No. 3,575,123 that the fluid resistance of a vessel can be reduced by coating its bottom with a water-insoluble hydrophilic polymer. Similar disclosures are found also in U.S. Pat. No. 3,515,370 and U.S. Pat. No. 3,635,756.
While conventional water-insoluble hydrophilic resin compositions are both water-absorptive and water-proof, they have a drawback in that an increase in water absorption is inevitably accompanied by a reduction in water resistance and vice versa. Although known polymers of 2-hydroxyethyl methacrylate have a water absorption capacity of 60-70% (weight % based on the dry weight of the polymer and hereinafter referred to as D.B.%) and excellent water-proof properties and, hence, have been put to various practical uses, they do not have practical utility as a coating composition for underwater constructions at the present time. The reason for this is that when applied as a coating to the bottom of a vessel requiring a high degree of water-proofing, they do not have sufficient coating strength to withstand water immersion and have only a poor bonding strength to the vessel body even though their effects for a resistance reduction in water can be recognized experimentally.
It has been known to insolubilize a hydrophilic polymer and further subject it to crosslinking to provide sufficient coating strength in water, but there are still some problems such as reduction in hydrophilic property caused by the crosslinking, impracticality of crosslinking conditions and the like. While the use of diacrylates or dimethacrylates such as ethyleneglycol diacrylate, ethyleneglycol dimethacrylate, propyleneglycol dimethacrylate and the like (acrylates and methacrylates are hereinafter referred to as (meth)acrylates), and that of dichromates such as ammonium dichromate, potassium dichromate and the like as crosslinking agents for polymers essentially comprising 2-hydroxyethyl methacrylate are disclosed, for example, in the above patents, they are not practical because di(meth)acrylates do not have a sufficient crosslinking effect when used in a low compounding ratio and, if used in a high compounding ratio, result in a significant reduction in water absorption and fragility in the coating film per se. The use of the dichromates requires ultra-violet rays for crosslinking which restricts crosslinking conditions in practical use and, additionally, provides a risk of environmental contamination caused by chromic substances.
U.S. Pat. No. 3,575,123 discloses a process for effecting crosslinking at ambient temperature by using a 2-component catalyst system, wherein a conventional peroxide catalyst and an amine promoter such as N,N-dimethyl aniline are used in combination. No practically satisfactory coating, however, can be obtained in view of coating strength and bonding strength in swelling.
Diisocyanates such as hexamethylene diisocyanate, methaphenylene diisocyanate and the like have excellent reactivity with hydroxyl groups which are often contained as side chains in hydrophilic polymers, but are of no practical importance as crosslinking agents for insolubilizing hydrophilic polymers to enhance the coating strength in water since they also react with polar solvents generally used as solvents for hydrophilic polymers.
Epoxy compounds generally used as crosslinking agents are also impractical since a high temperature and a long time are required for the reaction between them and the hydroxyl groups of the polymers of 2-hydroxyethyl methacrylate. While it is possible to add a catalyst for the reaction of the epoxy compounds to promote the reaction, there is no contribution to the crosslinking of the polymer containing the hydroxyl groups although the curing reaction is promoted, so that no improvement can be obtained in the coating strength in water when the polymer coating is submerged in water. Besides, the use of such catalysts cause other problems such as troubles in catalyst addition, leaching of unreacted catalyst and the like.
In view of the foregoing, no practical crosslinking methods have yet been found for polymers basically comprising 2-hydroxyethyl methacrylate. Accordingly, taking notice of epoxy compounds as general crosslinking agents, we have made a further study of polymers capable of producing a crosslinked polymer whose water absorption rate can be adjusted over a wide range and which has sufficient coating strength and bonding strength capable of withstanding longer water immersion as compared with the conventional polymers of 2-hydroxyethyl methacrylate.
It is, accordingly, an object of the present invention to provide a water-insoluble hydrophilic polymer having a water absorption rate adjustable over a wide range, and sufficient coating strength and bonding strength to withstand longer water immersion.
It is another object of the present invention to provide a water-insoluble hydrophilic polymer composition capable of crosslinking at room temperature to provide a coated article being water-absorptive and having a sufficient strength to withstand long periods of water immersion.